Discuss the different gaits used by bipeds and quadrupeds

This essay will outline and detail the various gaits used by bipeds and quadrupeds as well as the physical concepts which underlie these gaits using relevant examples of the animals which utilise them. The concept of what gait is and how these various gaits are classified will also be explained.
Gait should be thought of as the style with which an animal walks (as opposed to the act of putting one foot in front of the other), and as such the various gaits which may be used are very clearly defined. It is first useful to know some basic definitions which factor into gait classification.
Stride, simply defined as when one step is taken by each foot, is composed of both the stride frequency (amount of strides in a given time) and stride length (distance from one footfall to the next point this foot touches the ground). Temporal spacing relates the symmetry of the gait, determined by whether the legs move alternately or in unison.
The duty factor measures the fraction of time a foot is on the ground, generally when the duty factor is greater than 0.5 the gait is a walk and running gaits have a duty factor below 0.5. The relative phase is the time a foot is set down as a fraction of the total stride (Alexander, 1984).
Bipeds have four possible gaits: walking, running, skipping and hopping. Walking & running are symmetrical gaits where skipping & hopping are asymmetrical.
Humans favour walking as their primary gait. Walking is a highly efficient process, and requires less energy than the other human gaits. This efficiency stems from the way that the legs move, as two coupled pendula would. The physical basis for swinging pendula – that potential energy decreases in tandem with an increase in kinetic energy (and vice versa) holds true for walking as well. Thus the metabolic energy required for walking is minimised and no net mechanical work must be done while walking, meaning that only a small amount of energy is required to sustain walking (Kuo, Donelan, Ruina (2005)).
Additionally, the six determinants of gait as described by Inman et al (1981), work together to reduce the vertical and horizontal bobbing of the trunk by approximately half, helping to conserve energy (Whittle, 2002).
The Froude number, given by Fr = v2/gℓ (where v2 is the velocity, g is acceleration from gravity and ℓ is leg length) is dimensionless and can be used here to determine where walking becomes running. The maximum walking velocity is reached at a Froude number of 1.0 (this is without arching of the back, as is observed in competitive walking) however most bipeds will change to a run before this at the point where walking at this speed becomes ungainly (between a Froude of 0.5 and 1.0).

Faster motion requires running, a much more expensive form of motion than walking. While walking, one foot must always be in contact with the ground, however during running both feet must be in the air at some point. Running almost always requires more energy expenditure than walking (Hall et al, 2004).
The two remaining biped gaits are not typically used due to their inefficiency, and the fact that hopping would be more unstable than running. In the case of the Kangaroo however, which has much more powerful legs than a human, hopping is quite advantageous (with the drawback that moving slowly does not use less energy than moving quickly). When the legs are sufficiently strong to yield a great enough time in the air, hopping can be viable. Alternatively, in an environment with a lesser force of gravity (such as during the Apollo mission), humans can jump higher than normal and hopping becomes favoured over running (Alexander et al, 1992).
In humans, gait varies with age. Young children exhibit a wider walking base and a faster cadence than adults, (as well as walking without a heelstrike). Most of these differences can be attributed to the smaller stature of children, and ‘normal’ gait is usually observed by age 15. Conversely, older men (60 years and older) progress to a gait with a lesser stride length and decreased cadence (Whittle 2002).
Quadruped gaits are analogous to their biped counterparts. Quadrupeds may walk by ambling, run by trotting or pacing, skip by cantering or galloping and finally hop by bounding or pronking. As is true for bipeds, walking or running gaits are symmetrical while hopping and skipping gaits are asymmetrical. The gaits are classified by the relative phase of each foot, as detailed in Fig.1.
Not all quadrupeds are capable of all gaits, the available gaits depend on numerous factors (such as the length of the legs and the size of the feet) and gaits are changed depending on the size of the animal as well as the speed at which it is running. Horses for example typically trot at slow speed gradually switching to a canter and finally a gallop as speed increases. Horses and humans have been shown to choose gait based on the oxygen used by that gait for a given speed (Alexander, 1989). Camels are unique in that they pace rather than trot, whereas elephants only amble (Alexander, 1982).
In the aforementioned cases, equilibrium is not a problem as displacement from equilibrium can be corrected with every footfall when moving quickly. Animals like the tortoise, which have slow muscles and small feet, must maintain balance by keeping three feet on the ground at all times forming a ‘triangle of support’ (Alexander 1982).
In conclusion, it should be clear then that the scope of quadruped gaits is much greater than for bipeds; however the underlying principles which define gait apply to both. For example, all quadrupeds will change from a slow gait to a faster one at a similar Froude number regardless of their size and leg length, and all animals select that gait which will conserve the most oxygen.